Purchase new Windows 10 license or use Mac Boot Camp Partition?

I'm new using parallels and using a mac. I know I can download a partition of windows on the mac but the problem is I need visual studio and a lot of add ons. When I tried the first time I didn't create enough space, I used the min space but I want to be able to use Windows virtualization without using the boot partition to save space on my mac. So is there a way to do this, by purchasing a windows 10 product? Right Now I am using a trial version of windows 10 through Parallels, development mode but keeps telling me I need a license. Just not sure what I should do, like to keep using it like I am.

 

You can create the virtual machine directly on the external drive which will not use the space from your internal drive.